Owing to a recent growing demand for development of small vehicles, lightweight vehicles, and the improvement of silence, attempts are being made to manufacture a smaller and more lightweight electric auxiliary machine of vehicles, and to make the engine room airtight. On the other hand, there is a growing demand for a high output and a high efficiency for the performance of various apparatuses of the vehicle. Thus the present tendency is to compensate the miniaturization-caused reduction of the output of the electric auxiliary machine of the vehicle provided inside the engine room by rotating them at high speeds.
As examples of rolling bearings for use in the electric auxiliary of the vehicle, a rolling bearing for use in the fan coupling apparatus, a rolling bearing for use in the alternator, a rolling bearing for use in the idle pulley are outlined below.
Regarding the fan coupling apparatus, a viscous fluid is sealed inside it. A housing having an air-feeding fan mounted on its peripheral surface is coupled to a rotor directly connected to the engine through a bearing. By utilizing the shear resistance of the viscous fluid which increases and decreases in response to an atmospheric temperature, the fan-coupling apparatus controls the amount of a driving torque transmitted from the engine and the number of rotations of the fan, thereby feeding optimum air corresponding to the temperature of the engine. Therefore the rolling bearing for use in the fan-coupling apparatus is demanded to have a high heat resistance, grease-sealing performance, and durability so that the rolling bearing withstands a high rotational speed change in the range from 1000 rpm to 10000 rpm in dependence on a fluctuation of the temperature of the engine and in addition withstands very strict environment in which it is driven in summer at a high speed not less than 10000 rpm at a high temperature not less than 180° C.
The alternator for use in a car receives the rotation of an engine and generates electricity, thus supplying an electric power to an electric load of the car and has a function of charging a battery. Therefore the rolling bearing for use in the alternator is demanded to have a high heat resistance, grease-sealing performance, and durability so that the rolling bearing withstands a very strict environment in which it is rotated at a high speed not less than 10000 rpm at a high temperature not less than 180° C. As a method of improving the durability of the rolling bearing, there is proposed a material composed of the combination of urea-based grease and a terpolymer of vinylidene fluoride-tetrafluoroethylene-propylene or a bipolymer of tetrafluoroethylene-propylene for composing an elastic body of a sealing member demanded to have high heat resistance and durability (see patent document 1).
In the rolling bearing for use in a fan-coupling apparatus and an alternator proposed by the present applicant, grease containing an urea compound is used. The sealing member for sealing the grease is composed of the rubber molding of the vulcanizable fluororubber composition which contacts the grease. The rubber molding consists of the copolymer containing the monomer, for crosslinking use, containing tetrafluoroethylene, propylene, and unsaturated hydrocarbon, having 2 to 4 carbon atoms, in which a part of hydrogen atoms are substituted with fluorine atoms (see patent document 2).
The idle pulley is used as a belt tensioning part for the driving belt transmitting the engine power to the electric auxiliary machine of the vehicle. The idle pulley has the function of a pulley for imparting a tensile force to the driving belt when the distance between rotating shafts is fixed. The idle pulley has another function of an idler used to change the travel direction of the belt or preventing interference with obstacles to thereby reduce the volume of the engine chamber.
Therefore the rolling bearing for use in the idle pulley is demanded to have a high heat resistance, grease-sealing performance, and durability so that the rolling bearing withstands a very strict environment in which it is rotated at a high speed not less than 10000 rpm at a high temperature not less than 180° C. As a grease composition preferable for the rolling bearing which is used at high temperature and rotational speed, a grease composition is known (see patent document 3). The grease composition contains 0.5 to 10 wt % of amide-based wax having the property of preventing the base oil from being oxidized and having a melting point not less than 80° C. The base oil has a kinematic viscosity of 20 to 150 mm2/second at 40° C. The grease composition contains the urea-based thickener at 5 to 30 wt % for the entire grease composition.
In recent years, there is a tendency for small motors to be manufactured in industrial machines and for a bearing to be operated at a high surface pressure. In a servo motor, an acceleration and a deceleration to be applied thereto become high in stop—drive. As a result, the bearing has a large amount of slide. At a high temperature, the operation of high-speed drive—sudden deceleration drive—sudden acceleration drive—sudden stop is frequently repeated.
In a manufacturing line of a car, various kinds of industrial robots are used for assembling, welding, painting operations. To decrease a tact time for the improvement of productivity, there is a tendency for the motion speed of the robot to be increased. The operation of the robot is not successive, but intermittent. An increase in the operation speed of the robot causes the rolling bearing for use in a rotational portion to have an increase in the number of switching times of stop—start—operation—stopping operation per time. Each time switching is performed, an acceleration and a deceleration to be applied to the rolling bearing become high. Thereby the rolling bearing generates a large amount of slide.
In recent years, because the use condition for the rolling bearing for use in an electric auxiliary machine for a car, the bearing for use in a motor of an industrial machine, and the bearing for use in a robot has become strict, as described above, a peculiar peeling phenomenon occurs with the rolling surface of the bearing turning into white in its texture.
Unlike peeling which occurs in the inside of the rolling surface owing to metal fatigue, the peculiar peeling phenomenon occurs in the rolling surface at a comparatively shallow portion thereof with the rolling surface turning into white in its texture. That is, the peeling phenomenon is considered to be a hydrogen brittleness-caused destruction phenomenon.
As a method of preventing the peculiar peeling phenomenon which occurs in an early stage with the rolling surface turning into white in its texture, a method of adding a passivating agent to a grease composition and a method of adding bismuth dithiocarbamate thereto are known (see patent documents 4 and 5).
In recent years, because the rolling bearing for use in the electric auxiliary machine for a car, the motor of an industrial machine, and the robot are used in a strict condition, the method of adding the passivating agent to the grease composition and the like are incapable of solving the problem of the peeling phenomenon.
A fuel cell system attracts public attention as a new power source for a car or as a dispersion type generating apparatus. A fuel cell has a high output density and is operated at a low temperature. Further a material constructing the cell deteriorates to a low extent. A solid high molecular electrolyte type fuel cell which is actuated easily is effective as a power source of the car or the like.
In the fuel cell system, it is necessary to feed hydrogen and hydrogen rich modified gas serving as fuel and air serving as an oxidizing agent under pressure. To this end, various types of compressed fluid-feeding machines such as a supercharger, an impeller-type compressed fluid-feeding machine, a scroll-type compressed fluid-feeding machine, an iron plate-type compressed fluid-feeding machine, and a screw-type compressed fluid-feeding machine are used.
In the solid high molecular electrolyte type fuel cell, for power generation, hydrogen serving as fuel and air serving as an oxidizing agent react with each other in a chemical reaction to generate water. In addition, a high molecular film is humidified by a humidifier to function the high molecular film as a solid electrolyte. Thus the solid high molecular electrolyte type fuel cell is constantly kept in a water-containing state. Therefore water is contained in a gas fed under pressure by the compressed fluid-feeding machine. When the water penetrates into the bearing of the compressed fluid-feeding machine, metal contact occurs due to defective lubrication. Therefore as in the case of the rolling bearing for use in the electric auxiliary machine of the car, there is a case in which the hydrogen brittleness-caused peeling phenomenon occurs with the rolling surface of the bearing turning into white in its texture.
Because the use condition for the rolling bearing for use in the fuel cell system has also become strict, the method of adding the passivating agent to the grease composition (see patent document 4) and the method of adding the bismuth dithiocarbamate thereto (see patent document 5) are incapable of solving the problem of the hydrogen brittleness-caused peeling phenomenon. In compliance with a demand for an increase in the generation amount of electricity, the compressed fluid-feeding machine is requested to have high performance and operated at a high speed. Accordingly the rolling bearing is rotated at high speed with a large load applied thereto, which causes the bearing portion to have a high temperature of about 180° C. Thus the rolling bearing is demanded to have excellent heat resistance.
The fan-coupling apparatus, the alternator, and the like which are electric auxiliary machines for a car are used to efficiently utilize the rotational torque of an engine through a one-way clutch which is connected to an engine output shaft only when the engine is in a predetermined output state. The rotation-transmitting apparatus with a built-in one-way clutch mounted on the electric auxiliary machine for the car is frequently connected to and disconnected from the engine output shaft. When the rotation-transmitting apparatus with the built-in one-way clutch is connected to the engine output shaft, it has a high rotational speed. Consequently a high load is applied to the rotation-transmitting apparatus with the built-in one-way clutch, and it generates a large amount of heat and vibration. The use condition for the rotation-transmitting apparatus with the built-in one-way clutch has become strict as the car has come to have high performance and output.
Because the use condition for the rotation-transmitting apparatus with the built-in one-way clutch has become strict, when rolling bearings rotate, namely, when a roller clutch overruns, the hydrogen brittleness-caused peeling phenomenon is liable to occur with the rolling contact surfaces of balls and the rolling surfaces of inner and outer rings constructing the rolling bearing turning into white in the textures thereof. In addition, when the rolling bearings do not rotate, namely, when the roller clutch is locked, contact portions between the rolling contact surfaces of the balls and the rolling surface of each of the inner and outer rings are liable to be fretted. Therefore grease which is enclosed in a space where the balls are mounted is desired to prevent the generation of the peeling and fretting wear.
The electric auxiliary machine on which the rotation-transmitting apparatus with the built-in one-way clutch is mounted is frequently installed on a lower portion of an engine room. Thus while a car is travelling, rainwater is liable to penetrate into the rotation-transmitting apparatus with the built-in one-way clutch. When the rainwater penetrates into the space in which the balls of rolling bearings are mounted, the rolling contact surfaces of the balls and the rolling surfaces of the inner and outer rings are liable to be corroded. Therefore the grease which is enclosed in the space in which the balls of rolling bearings are mounted is required to have rust-preventing performance superior to that of grease which is used in other portions.
In the one-way clutch of the rotation-transmitting apparatus with the built-in one-way clutch for use in the alternator, the one using grease containing ether oil as its base oil (see patent document 6), the one using grease whose viscosity pressure coefficient is not less than a predetermined value (see patent document 7), and the one using grease containing synthetic oil whose kinematic viscosity at 40° C. is not more than 60 mm2/second as its base oil (see patent document 8) are known.
But in patent document 6, the one-way clutch using the grease containing the ether oil as its base oil has a problem that it has an insufficient low frictional wear property when the one-way clutch is in an overrun state. In the patent document 7, the use of the grease whose viscosity pressure coefficient is not less than the predetermined value is effective for securely realizing a locked state, but has a problem that the grease is incapable of sufficiently restraining wear in the overrun state. In the patent document 8, the grease containing the synthetic oil whose kinematic viscosity is low as its base oil has an insufficient heat resistance. Thus it is difficult to use the grease for a long time.
In a wheel-supporting apparatus for supporting non-drive front wheels of a rear-wheel-drive vehicle, two rolling bearings are mounted on an axle (knuckle spindle) provided on a steering knuckle, a flange is formed on an outside surface of the axle hub rotatably supported by the rolling bearings, and a brake drum and wheel disks of wheels are mounted with a stud bolt provided on the flange and with a nut which engages the stud bolt through a screw. A back plate is mounted on a flange formed on the steering knuckle to support a braking mechanism for imparting a braking force to the braking drum.
In the wheel-supporting apparatus, as a rolling bearing rotatably supporting the axle hub, a tapered roller bearing having a large load-carrying capacity and a high rigidity is used. The tapered roller bearing is lubricated with grease charged between the axle and the axle hub.
Because the bearing for use in the wheel-supporting apparatus is used in the strict condition in which it is operated at a high speed with a large load applied thereto, a bearing ring rib makes a sliding motion on large end faces of rollers and a rib portion, a lubricating oil film consisting of the lubricating grease is liable to be broken. When the lubricating oil film is broken, metal contact occurs and there is an increase in heat generation and frictional wear. Therefore it is necessary to improve the lubricating property and withstand load of the bearing in the condition in which the bearing is rotated at a high speed with a large load applied thereto and prevent the metal contact caused by the breakage of the lubricating oil film. To this end, grease containing an extreme pressure agent is used to decrease the disadvantage.
As an example of the bearing for use in the wheel-supporting apparatus to which a high load is applied at a high-speed operation, a bearing for use in the rolling stock in which grease containing not more than 20 wt % of an organic metal compound containing metal selected from among nickel, tellurium, selenium, copper, and iron for the entire amount of the grease is enclosed is known (see patent document 9).
But as the use condition for the roller bearing has become strict as in the case of lubrication of the roller bearing in a high-speed condition of not less than 100000 in dN value, the use of the conventional grease makes it difficult to use the roller bearing. In the roller bearing for use in the wheel-supporting apparatus, rolling friction is generated between the rolling surfaces of the inner and outer rings and “rollers”, and a sliding friction is generated between a rib portion and the “rollers”. The sliding friction is larger than the rolling friction. Thus when the use condition is strict, the rib portion is liable to be seized. Consequently it is necessary to perform a grease-replacing work frequently. Thus it is impossible to accomplish a maintenance-free operation.
In addition, in a hydraulic motor, a hydraulic pump, and an axle planetary gear portion, bearings lubricated with gear oil and hydraulic oil are generally used. As these oils, mineral oil and water-glycol hydraulic oil are generally used.
As the use condition for the bearing lubricated with the gear oil and the hydraulic oil has become strict in recent years by operating it at a high speed with a large load applied thereto, similarly to the use of the grease, the hydrogen brittleness-caused peeling phenomenon occurs in an early stage with the rolling surface of the bearing turning into white in its texture.
The hydrogen causing the peeling phenomenon to occur is generated by decomposition of lubricant oil. The hydrogen is generated from the lubricating oil for the following two reasons: (1) decomposition owing to heat and shear and (2) decomposition reaction which takes place with a metal surface newly generated by wear acting as a catalyst. The generated hydrogen penetrates into bearing steel easily, thus causing the peeling to occur owing to the hydrogen brittleness. Which of (1) and (2) affects the generation of the hydrogen to a higher extent depends on a use condition for the bearing. When the generation of the hydrogen is caused mainly by (2), reducing the time period in which the metal surface newly generated by wear is exposed is effective for restraining the generation of the hydrogen. That is, adding an additive which makes the newly generated metal surface inactive immediately to the lubricating oil is effective for restraining the generation of the hydrogen.
As a measure for preventing the occurrence of the hydrogen brittleness-caused peeling phenomenon based on the above-described knowledge, a method of adding a metal salt of nitric acid to base grease (see patent document 10) and a method of adding molybdate to base grease (see patent documents 11 and 12) are known.
But as the recent use condition for the rolling bearing becomes strict, a method of only adding these additives is becoming insufficient.
Patent document 1: Japanese Patent Application Laid-Open No. 2001-65578
Patent document 2: Japanese Patent Application Laid-Open No. 2005-256891
Patent document 3: Japanese Patent Application Laid-Open No. 2003-105366
Patent document 4: Japanese Patent Application Laid-Open No. 3-210394
Patent document 5: Japanese Patent Application Laid-Open No. 2005-42102
Patent document 6: Japanese Patent Application Laid-Open No. 11-82688
Patent document 7: Japanese Patent Application Laid-Open No. 2000-234638
Patent document 8: Japanese Patent Application Laid-Open No. 2000-253620
Patent document 9: Japanese Patent Application Laid-Open No. 10-17884
Patent document 10: Japanese Patent Application Laid-Open No. 2005-29623
Patent document 11: Japanese Patent Application Laid-Open No. 2005-29622
Patent document 12: Japanese Patent Application Laid-Open No. 2005-112901